Debris threat prompts space station crew to evacuate

In a rare move, the three crew members of the International Space Station briefly evacuated the station and took shelter in a docked Soyuz spacecraft today due to the threat of a possible collision with a piece of space junk, NASA says.

The agency says the threat has now passed and the crew will return to the station, but the last-minute move highlights how difficult it is to determine the paths of space debris that follow certain kinds of orbits.

Tracking data suggested that the debris, a 13-centimetre-wide piece of a spent satellite motor called PAM-D (unrelated to debris from last month's satellite crash), could come within about 4.5 kilometres of the station at 1239 EDT (1639 GMT).

That sounds like a safe enough distance, but it was too close for NASA's comfort. That's because there was a lot of uncertainty in the measurement due to the elongated orbit of the debris, says Gene Stansbery, NASA's orbital debris programme manager. "They weren't confident of that 4.5-km miss distance," he told New Scientist.

And the consequences of a possible collision could have been dire. If the debris hit a pressurised module in the station, "it could punch a hole in it" and cause the module to lose pressurisation critical to the crew's survival, Stansbery says.

"If you had something the size and weight of a nickel coin going at 10 km/s" – the estimated relative speed between the station and the PAM-D debris – "then it has the equivalent energy of a small car at 50 miles per hour (80 km/h)," he says. "Thirteen centimetres is much larger than that."

Little warning

The possible collision was only discovered on Wednesday night, Stansbery says – not enough time to move the space station away from the debris.

As a result, at 1140 EDT (1540 GMT) on Thursday, crew members Mike Fincke, Yury Lonchakov and Sandra Magnus were told that they would be taking refuge in the Soyuz, a docked Russian spaceship that could bring the crew back to Earth in case of an emergency on the station.

The three made their way to the Soyuz so they could quickly close the hatches in case the debris damaged the station and they had to undock. Such evacuations are "very rare", NASA spokesman William Jeffs told New Scientist.

At 1245 EDT (1645 GMT), NASA says the crew was notified that the threat had passed and told to return to the station. But the temporary evacuation raises questions about why NASA had such little warning before the potential collision.

Elongated orbit

Stansbery says the orbit of the debris is to blame. The debris is a weight that was jettisoned into space after it was used to slow the spin of a satellite that launched in 1993. He says the debris might have a mass of a couple of kilograms.

The debris travels on a very elongated, or eccentric, orbit around Earth, coming within about 150 km of the planet and venturing as far away as about 4200 km.

Because it comes so close to Earth, it goes through a fairly significant amount of the Earth's atmosphere, says Stansbery. "How much atmospheric drag it gets is not something that can be predicted very well," he says. "It's hard to predict from one orbit to the next where the object's going to be."

Moreover, the fact that it travels so far away from Earth also makes it hard to track. That's because ground-based radar dishes essentially see objects within a "cone" of a certain size around Earth.

When the object is far away in its orbit, it is harder to detect with radar because it goes outside that cone. "It doesn't come within the field of view of radars as often as something in a circular orbit would," he says. "There are fewer tracking opportunities."

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Given enough notice of a possible collision, the space station can fire thrusters to move out of the way - eight such 'avoidance manoeuvres' have been made in the past. But on Thursday, the threat of a collision was discovered too late to move the station, so the crew took shelter in a docked Soyuz spacecraft (Image: NASA)